The present invention relates to phased array radar technology, and, more particularly, relates to a space-fed phased array radar antenna employing the window shade deployment technique for use in a space-based radar.
The current state-of-the-art technology includes a window shade deployed space-fed phased array radar antenna which is particularly suited for use in space. The rolled antenna is advantageous because it minimizes storage space aboard a spacecraft, such as a satellite, where available space is at a minimum. This is because, when in a stowed position, it minimizes the amount of space required in a launch vehicle. When the spacecraft achieves a selected orbit, the antenna is deployed and the window shade structure is unrolled to a fully expanded operative condition. Such an antenna may consist of a low to medium power RF feed which illuminates a lens aperture membrane. Active transmit/receive (T/R) modules in the aperture membrane receive radar pulses from the feed, perform beam-steering phase shifts, amplify them and re-transmit the signal towards a target of interest on the ground or in the air.
The reflected energy is received in reverse order, being amplified by the T/R modules and then focused back into the space feed. Radar processors and the supporting subsystems are part of the satellite bus and may be located in the feed or at the base of a feed mast. In line with current technology, the aperture consists of a tensioned membrane consisting of three separate equally spaced layers, which provides for a very lightweight, yet sufficiently flat, aperture plane. Array flatness requirements for the space-fed approach are less severe than for corporate-fed approaches by an order of magnitude. The membrane aperture can be rolled up onto a drum, resulting in a simple, compact, and repeatable method for deployment and retraction of the antenna.
Although this technique may seem appropriate, there are several potential problems with the technology. First of all, although the three-layer membrane conforms to the weight and flatness requirements necessary for a space-fed lens space-based radar, it is extremely complex to manufacture. This is due primarily to the three separate layers inherent in the design, as well as the many different parts and connections necessary between these parts. The layers are equally spaced on depoloyment (separated by 1/4 wavelength) and come together when stowed. Electronics embedded in the middle layer must be connected to the antenna elements on the outside layers. This also adversely affects the performance reliability of the radar. Another area of concern is the type of antenna radiating element used--a dipole. The antenna pattern of a monopole is much more applicable for a space-based radar mission. Thus, there is a need to develop a monopole antenna structure for use in space-based radars.